1. Field of the Invention
This invention relates to a process for making a sensor for detecting a fluid flow velocity or flow amount by utilizing a dependency of resistance of a platinum resistor upon temperature.
2. Brief Description of the Prior Art
It is possible to detect a fluid flow velocity of flow amount by utilizing a dependency of resistance upon temperature, and this has been practically utilized as a hot wire sensor for detecting a fluid flow velocity or flow amount, where the sensor is heated up to 100.degree.-200.degree. C. by passing an electric current through the sensor, and when a fluid comes in contact with a sensor, the heat is carried away by the fluid from the sensor in such an amount as to correspond to the fluid flow velocity or flow amount, and consequently the resistance of sensor is changed. That is, a fluid flow velocity or flow amount can be detected from such a change in resistance of sensor. Generally, a platinum wire or tungsten wire is used as the resistor.
The conventional hot wire sensor for detecting a fluid flow velocity of flow amount can be classified into the following three major groups: (1) coil type, (2) fine wire type and (3) a thin film type. The coil type (1) is a platinum wire, etc. wound around a glass bar, etc., and has a high mechanical strength, but it is difficult to uniformly and densely wind the wire around the glass bar. Thus, the product yield is deteriorated and the production cost is high. Furthermore, the wire diameter is not always uniform, so that the precision of resistance is not so high. The fine wire type (2) is platinum wires or tungsten wires having a diameter as small as a few .mu.m connected to between lead wires, and has a low resistance to mechanical shock, and when fouled, it will have a much lower resistance to mechanical shock due to an increased weight on the sensor part. Furthermore, the wire diameter is not always uniform, and adjustment of wire length is difficult to effect due to their processability, so that the precision of resistance is low, and the production cost is high.
The thin film type (3) is a platinum film formed at the tip end of quartz bar by vapor deposition, etc. and it is difficult to obtain a film of uniform thickness, so that the precision of resistance is low. Furthermore, the vapor deposition and other relevant processing technique are not suitable for mass production. That is, the thin film type (3) is not suitable for the mass production, and also the production cost is high.
Recently, such a resistor for detecting temperature has been proposed, which is prepared by providing a platinum film spirally on the surface of an aluminum tube by coating, inserting an electrode into the tube, connecting the platinum film to the electrode through a conductor paste, and covering the platinum film with crystallizable glass. Application of such a resistor as a sensor for detecting a fluid flow velocity or flow amount has the following disadvantages (1)-(3).
(1) Since a conductor film is prepared by applying a uniform mixture of platinum powder and glass powder in an organic vehicle, followed by firing at less than 1,100.degree. C., the growth of platinum particles is suppressed, and a platinum film with stable characteristics is hard to obtain. Furthermore, a change in characteristics with time is large.
(2) Crystallizable glass for covering the conductor film is less gas-tight than non-crystallizable glass, and the film must be made thicker, so that a response speed is lowered.
(3) Outer diameter is as large as 3 mm, and the shape is not suitable for detecting a fluid flow velocity or flow amount.